Our Milky Way is home to at least 17 billion planets that are similar in size to Earth, a new estimate suggests. That's more than two Earth-size planets for every person on the globe.

Just how many are located in the sweet spot where water could exist is "simply too early to call," said Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, who presented his work at an astronomy meeting Monday.

It's the first reliable tally of the number of worlds outside the solar system that are the size of Earth, but the hunt for our twin is far from over.

Despite the explosion of exoplanet discoveries in recent years, one find remains elusive: A planet that's not only the right size but also in the so-called Goldilocks zone where it's not too hot or too cold for water to be in liquid form on the surface.

Astronomers narrow down habitable zone

The sheer number of Earth-size planets gives astronomers a starting point to narrow down which ones are in the habitable zone.

Fressin and his team came up with their figure by conducting a fresh analysis of data collected by NASA's Kepler spacecraft, which was launched in 2009 to track down other Earths. They estimated at least one in six stars in the galaxy hosts a planet the size of ours, translating to at least 17 billion Earth-size worlds.

Using a different method, a team from the University of California, Berkeley and University of Hawaii separately came up with a similar estimate. They calculated 17 per cent of distant stars have planets that are the same size as Earth or slightly larger.

The findings were presented at the American Astronomical Society in Long Beach, Calif.

Meanwhile, the Kepler spacecraft continues to spot planets as they pass between Earth and the star they orbit. It found 461 new candidate planets, bringing the total to 2,740 potential planets, said mission scientist Christopher Burke at the SETI Institute.

Most of the new Kepler finds were driven by discoveries of Earth-size planets and super-Earths. Four of those are thought to reside in the Goldilocks zone, but more observations are needed.

Fressin said it's clear that rocky planets abound outside the solar system.

"If you look up on a starry night, each star you're looking at -- almost each one of them -- has a planetary system," Fressin said.